proc delete_line {filename prefix} {
   set chan [open $filename]
   set data ""
   while {[gets $chan line] != -1} {
      if {[string equal -length [string length $prefix] $line $prefix]} {
         break
      } else {
         append data $line\n
      }
   }
   append data [read $chan]
   close $chan
   set chan [open $filename w]
   puts -nonewline $chan $data
   close $chan
}